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Heat Treatment of Steels

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1 Heat Treatment of Steels
MSE 201Lab IV

2 Samples AISI-SAE 1018 – 0.18 % C AISI-SAE 1045 – 0.45 % C
Austenitized at 870°C for 2 hours

3 Heat Treatments A – Furnace Annealed – Slow cooled
N - Normalized - Air cooled. O - Oil Quenched WQ – Water quenched. WT(370)– Water quenched, tempered at 370°C for 1 hour. WT(705)– Water quenched, tempered at 705°C for 1 hour.

4 Jominy Test Generally, the faster steel cools, the harder it will be. The Jominy bar measures the hardenbility of a steel Softest Hardest

5 Proceed to Furnace Room to:
Quench the samples (except the normalized ones) Place the tempering samples into Furnaces Jominy Test demonstration

6 Pearlite Formation Austenite precipitates Fe3C at Eutectoid Transformation Temperature (727°C). When slow cooled, this is Pearlite (looks like Mother of Pearl)

7 Diffusion of Carbon in Pearlite

8 Morphology of Pearlite
(b) (a) coarse pearlite (b) fine pearlite 3000X

9 What About Cooling Rates?
Faster cooling gives “non-equilibrium microconstituents”… Bainite Martensite And more! To know what microconstituents are present, you must look at cooling curve diagrams

10 Microconstituents vs. Cooling Rate
Spheroidite: Spherical “globs” of Fe3C in Ferrite Pearlite: Layers of  ferrite and Fe3C Course Pearlite Fine Pearlite Bainite: 200 – 500 C Transformation Martensite: Rapid Cooling In creasing Cooling Rate

11 Bainite Upper (550-350°C) Rods of Fe3C Lower (350-250°C)
Fe3C Precipitates in Plates of Ferrite It is still Ferrite and Cementite! It’s just acicular.

12 Martensite Diffusionless transformation of FCC to BCT (more volume!)
Lenticular structure Very hard & very brittle.

13 TTT Diagrams

14 Full TTT Diagram The complete TTT diagram for an iron-carbon alloy of eutectoid composition. A: austenite B: bainite M: martensite P: pearlite

15 So What’s a CCT Diagram? Phase Transformations and Production of Microconstituents takes TIME. Higher Temperature = Less Time. If you don’t hold at one temperature and allow time to change, you are “Continuously Cooling”. Therefore, a CCT diagram’s transition lines will be different than a TTT diagram.

16 Slow Cooling Time in region indicates amount of microconstituent!

17 Medium Cooling Cooling Rate, R, is Change in Temp / Time °C/s

18 Fast Cooling This steel is very hardenable… 100% Martensite in ~ 1 minute of cooling!

19 What is Tempering? Martensite needs to be tempered to get better ductility. This happens when Fe3C is allowed to precipitate from the supercooled Martensite.

20 Spheroidite If tempered for a long time, Fe3C forms “spheres” and grows inside Ferrite. Very soft, easy to machine

21 Tempering Demonstration
Observe Steel Wire Experiment… What causes wire to sag on heating? When cooling, wire gets tight, then sags again. Why? Why does steel snap like chalk when cooled fast, but tempering restores “strength”?

22 So What is “Hardenability”?
Jominy Bar used to show how cooling rate affects hardness Alloyed steels (Cr, Mo, Ni, etc.) have higher hardenbility at same cooling rates than carbon steels

23 Typical Jomminy Curves
4340: Very hardenable, More expensive 1040: Less hardenable, Less expensive

24 Go Obtain Tempered Samples
Why can we quench the tempered samples (at 705 and 370°C) in water? Will new Martensite form? Grind oxide scale before measuring hardness Convert data to DPH Turn datasheet to TA

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